The invention relates to a device for measuring physiological information through use of photoplethysmograms measured at a plurality of wavelengths, and more particularly, to an improvement in a pulse photometer used for diagnosing a problem in a respiratory system or a circulatory system.
A known technique for measuring a concentration of light absorbing materials contained in blood during medical care is pulse photometry intended for measuring an oxygen saturation SpO2, the concentration of abnormal hemoglobin such as carboxyhemoglobin (COHb) and methemoglobin (MetHb), or a concentration of injected pigment. A device for measuring oxygen saturation SpO2 in particular is called a pulse oximeter.
The principle of the pulse oximeter is to irradiate onto living tissue light rays of a plurality of wavelengths exhibiting different light absorbing characteristics with respect to a material of interest and to determine the concentration of the material on the basis of a pulse wave produced through consecutive measurement of quantity of transmitted light.
Japanese Patent No. 3270917 (cf., claims 1, 2 and FIGS. 2 and 4) discloses determination of a saturation level of oxygen in arterial blood and a concentration of light-absorbing materials in the same by: emitting two light-rays of different wavelengths onto living tissue; plotting graphs with levels of the resultant two pulse waves originating from transmitted light being taken as vertical and horizontal axes, respectively, to thereby determine regression lines; and determining the oxygen saturation and the concentration of light-absorbing materials in the arterial blood on the basis of gradients of the regression lines. This technique enhances measurement accuracy and reduces power consumption.
However, a large amount of computing operation is still required to determine regression lines and gradients thereof through use of many sampled data sets pertaining to pulse waves of respective wavelengths.
The principle will now be described by taking, as an example, a pulse oximeter for measuring the oxygen saturation in arterial blood.
This technique is not limited to a pulse oximeter but is also applicable to a device (or a pulse photometer) which, on the basis of the principle of the pulse photometry, measures abnormal hemoglobin (carboxyhemoglobin or methemoglobin) or a light absorbing material in blood such as pigment injected in blood.
FIG. 1 is a block diagram of such a pulse oximeter. The device will now be described by taking the pulse oximeter as an example.
At the time of measurement, a probe 1 is usually attached to an area subjected to the measurement.
A processor 8 produces light emission timings for a light emitter (LEDs) 2 which produce light rays of different wavelengths, that is, a red-light emitting diode (hereinafter abbreviated “R-LED”) and an infrared-light-ray emitting diode (hereinafter abbreviated “IR-LED”). The R-LED and the IR-LED, both being provided in the probe 1, are driven by an LED driver 4, to thereby alternately emit light.
The light emitted by the LEDs 2 passes through the area subjected to the measurement to which the probe is attached, and is received by a light receiver (photo diode; PD) 3 provided in the probe 1.
The thus-received signal is converted from light into electricity by the light receiver 3 and is subsequently converted into a voltage by an input section 5.
Components reflecting optical characteristics of pulsation of the area subjected to the measurement appear in this signal in the form of AC components.
By a demodulator 6, the signal output from the light receiver (PD) 3 is separated and demodulated into a waveform of an infrared light ray (IR) and that of a red light ray (R). The waveforms are digitized into signal trains by an A/D converter 7. The signal trains are sent to the processor 8, where the signal trains are processed, thereby computing, e.g., oxygen saturation SpO2.
The signal trains that have been digitized by the A/D converter 7 corresponding to infrared light (IR) and red light (R) form respective measured data sets.
Light absorbance A of the light that has passed through tissue is expressed by Equation (1), with the attenuation of light in blood being taken as Ab and the attenuation of light in other tissue being taken as At.A=Ab+At=ln (Iin)−ln (Iout)=ECD+At  (1)wherein:
Iout: Intensity of transmitted light
Iin: Intensity of incident light
E: Absorption coefficient of blood
C: Concentration of hemoglobin in blood
D: Thickness of blood
When the blood thickness has assumed D+ΔD as a result of pulsation of blood, Equation 1 is transformed as follows.A+ΔAb=ln (Iin)−ln (Iout)+Δln (Iout)=EC(D+ΔD)+At  (2)Subtracting Equation 1 from Equation 2, Equation 3 is obtained.ΔAb=Δln (Iout)=ECΔD   (3)Equation 3 is defined for wavelengths λ1, λ2 as follows.ΔAb1=−Δln (Iout1)=E1CΔD  (4)ΔAb2=−Δln (Iout2)=E2CΔD  (5)Subscripts 1, 2 indicate that the corresponding terms are relevant to wavelengths λ1, λ2. Provided that Equation 4/Equation 5=Φs, we have
                              Φ          s                =                                            Δ              ⁢                                                          ⁢              A              ⁢                                                          ⁢                              b                1                                                    Δ              ⁢                                                          ⁢              A              ⁢                                                          ⁢                              b                2                                              =                                                    Δ                ⁢                                                                  ⁢                                  ln                  ⁡                                      (                                          I                      ⁢                                                                                          ⁢                                              out                        1                                                              )                                                                              Δ                ⁢                                                                  ⁢                                  ln                  ⁡                                      (                                          I                      ⁢                                                                                          ⁢                                              out                        2                                                              )                                                                        =                                          E                1                                            E                2                                                                        (        6        )            
Φ is determined by measuring Δln(Iout1), Δln(Iout2), whereby the oxygen saturation is determined as follows.SpO2=f(Φs)  (7)
When the influence of body movement or a like factor other than the component reflecting pulsation is superimposed on Δln(Iout1), Δln(Iout2), light absorbance of each of the wavelengths is defined as follows.ΔAb1′=−Δln (Iout1′)=E1CΔD+En1CnΔDn  (8)ΔAb2′=−Δln (Iout2′)=E2CΔD+En2CnΔDn  (9)wherein:
En: Absorption coefficient of the noise source
Cn: Concentration of the noise source
Dn: Change in thickness of the noise source
A waveform (pulse wave) reflecting a pulsation component is not solely obtained, and a signal on which noise is superimposed is observed. In this case, a light absorbance ratio Φ′ is defined as follows:
                              Φ          ′                =                                            Δ              ⁢                                                          ⁢              A              ⁢                                                          ⁢                              b                1                ′                                                    Δ              ⁢                                                          ⁢              A              ⁢                                                          ⁢                              b                2                ′                                              =                                                    Δ                ⁢                                                                  ⁢                                  ln                  ⁡                                      (                                          I                      ⁢                                                                                          ⁢                                              out                        1                        ′                                                              )                                                                              Δ                ⁢                                                                  ⁢                                  ln                  ⁡                                      (                                          I                      ⁢                                                                                          ⁢                                              out                        2                        ′                                                              )                                                                        =                                                                                                      E                      1                                        ⁢                    C                    ⁢                                                                                  ⁢                    Δ                    ⁢                                                                                  ⁢                    D                                    +                                                            En                      1                                        ⁢                    Cn                    ⁢                                                                                  ⁢                    Δ                    ⁢                                                                                  ⁢                    Dn                                                  ⁢                                                                                                                                    E                    2                                    ⁢                  C                  ⁢                                                                          ⁢                  Δ                  ⁢                                                                          ⁢                  D                                +                                                      En                    2                                    ⁢                  Cn                  ⁢                                                                          ⁢                  Δ                  ⁢                                                                          ⁢                  Dn                                                                                        (        10        )            Hence, the light absorbance ratio does not coincide with the oxygen saturation in arterial blood.
FIG. 2A shows waveforms which have been subjected to: processing in which transmitted light intensity data at respective wavelengths at predetermined time intervals are logarithmic-computed; and then a mean of the logarithmic-computed values is made to zero or high-pass filtering is performed with respect to the logarithmic-computed values.
FIG. 2B shows a graph through use of waveform data measured substantially at the same time when the waveforms shown in FIG. 2A are measured (the time is sufficiently shorter than the duration of a frequency component of a pulse wave) with the amplitude of infrared light being taken as a horizontal axis and the amplitude of red light being taken as a vertical axis.
If the observed data reflect a mere pulse wave component, the graph essentially assumes the shape of a straight line. The gradient of the straight line shows the light absorbance ratio Φs.
However, when noise is superimposed on the data as shown in FIG. 3A, there is obtained a result Into which the light absorbance ratio of noise and the light absorbance ratio of pulsation are merged as shown in FIG. 3B, in contrast with the case of a pulse wave component.
As mentioned above, when measured pulse wave data include noise, an accurate light absorption ratio cannot be measured, and hence noise must be reduced.
Hitherto-known techniques for reducing this noise are a frequency analysis technique and an independent component analysis technique.
However, when the frequency of noise overlaps the fundamental wave of a signal component of measured pulse wave data or a harmonic wave of the same, the frequency analysis technique encounters difficulty in extracting a signal component.
According to the independent component analysis technique, an observed signal cannot be dissolved into independent components beyond measured waveform data. When a plurality of noise sources are present, difficulty is encountered in dissolving the data.